1. Field of the Invention
The present invention relates to a process for preparing melt-processable aromatic polyesters via the direct condensation of aromatic hydroxy acids. More specifically, the present invention relates to a process of direct polymerization of aromatic hydroxy acids which is conducted in the presence of a Group IV or V metallic catalyst.
2. Description of the Prior Art
Wholly aromatic polyester resins, with the aromatic polyester being considered to be "wholly" aromatic in the sense that each moiety present contributes at least one aromatic ring to the polymer backbone, have long been known. For instance, 4-hydroxybenzoic acid homopolymer and copolymers have been provided in the past and are commercially available. Those wholly aromatic polyesters normally encountered in the prior art have tended to be somewhat intractable in nature and to present substantial difficulties if one attempts to melt process the same while employing conventional melt processing procedures. Such polymers commonly are crystalline in nature, relatively high melting or possess a decomposition temperature which is below the melting point, and when molten frequently exhibit an isotropic melt phase. Molding techniques such as compression molding or sintering may be utilized with such materials; however, injection molding, melt spinning, etc., commonly have not been viable alternatives or when attempted commonly have been accomplished with difficulty.
Representative publications which discuss wholly aromatic polyesters include: (a) "Polyesters of Hydroxybenzoic Acids," by Russell Gilkey and John R. Caldewell, J. of Applied Polymer Sci., Vol. II, Pages 198 to 202 (1959), (b) "Polyarylates (Polyesters From Aromatic Dicarboxylic Acids and Bisphenols)," by G. Bier, Polymer, Vol. 15, Pages 527 to 535 (August 1974), (c) "Aromatic Polyester Plastics," by S. G. Cottis, Modern Plastics, Pages 62 to 63 (July 1975); and (d) "Poly(p-Oxygenzoyl Systems): Homopolymer for Coatings: Copolymers for Compression and Injection Molding," by Roger S. Storm and Steven G. Cottis, Coatings Plast. Preprint, Vol. 34, No. 1, Pages 194 to 197 (April 1974). See also, U.S. Pat. Nos. 3,039,994; 3,169,121; 3,321,437; 3,553,167; 3,637,595; 3,651,014; 3,723,338; 3,759,870; 3,767,621, 3,778,410; 3,787,370; 3,790,528; 3,829,406; 3,890,256; and 3,975,487.
Also, it has been disclosed that certain polyesters may be formed which exhibit melt anisotropy. See for instance, (a) "Polyester X7G-A Self Reinforced Thermoplastic," by W. J. Jackson Jr., H. F. Kuhfuss, and T. F. Gray, Jr. 30th Anniversary Technical Conference, 1975 Reinforced Plastic/Composites Institute. The Society of the Plastics Industry, Inc., Section 17-D, Pages 1 to 4, (b) Belgian Pat. Nos. 828,935 and 828,936, (c) Dutch Pat. No. 7505551, (d) West German Nos. 2520819, 2520820, 2722120, 2834535, 2834536 and 2834537, (e) Japanese Nos. 43-223; 2132-116; and 3021-293, (f) U.S. Pat. Nos. 3,991,013, 3,991,014; 4,057,597; 4,066,620; 4,067,852; 4,075,262; 4,083,829; 4,118,372; 4,130,545; 4,130,702; 4,156,070; 4,159,365; 4,169,933; 4,181,792; 4,183,895; 4,188,476; 4,189,996; 4,201,856; 4,224,433; 4,226,970; 4,230,817; 4,232,143; 4,232,144; 4,238,598; 4,238,600; 4,239,599; 4,242,496; 4,245,082; 4,245,084; 4,247,514; 4,256,802; 4,269,965; 4,287,332 and 4,294,955; and (g) U.K. application No. 2,002,404.
Additionally in commonly assigned U.S. Pat. No. 4,161,470 is claimed a polyester of 6-hydroxy-2-naphthoic acid and para-hydroxy benzoic acid, and in commonly assigned U.S. Pat. No. 4,219,461, is claimed a polyester of 6-hydroxy-2-naphthoic acid, para-hydroxybenzoic acid, aromatic diol, and aromatic acid. Moreover, in commonly assigned U.S. Pat. No. 4,256,624 is claimed a polyester of 6-hydroxy-2-naphthoic acid, an aromatic diol and an aromatic diacid. Each of these polyesters exhibits an anisotropic melt phase and is capable of readily undergoing melt processing to form quality fibers, molded articles, etc.
An established procedure for preparing aromatic polyesters of fiber-forming molecular weight is by thermally reacting acetates with carboxylic acids. In other words, the aromatic hydroxy groups are acetylated. The use of acetylated reactants has been of utmost importance when preparing a polyester of an aromatic hydroxy acid since certain aromatic hydroxy acid compounds tend to decarboxylate upon direct esterification. For example, para-hydroxybenzoic acid is notorious for severe decarboxylation upon attempted direct esterification. Decarboxylation adversely effects the yield of the process, the nature and composition of the final product, and the economic viability of the process.
A reaction involving acetylated reactants, however, liberates acetic acid which is corrosive and thus requires the use of special corrosion resistant equipment. A further disadvantage is that the esterified or acetylated reactants are more costly than the unesterified reactants. Accordingly, a polyesterification involving an aromatic hydroxy acid wherein all hydroxy and acid moieties are efficiently and effectively reacted directly would be of advantage since the high cost of using esterified products would be avoided and by-product water presents far less of a disposal problem than the acetic acid involved with ester reactants.
The polycondensation of a phenolic moiety with an aromatic carboxylic acid moiety, in general, has been found to be impracticable or simply not to work. However, some limited success has been achieved.
For example, U.S. Pat. No. 4,093,595 discloses a catalytic procedure for the preparation of polyesters from (1) dihydric phenols and (2) aromatic dicarboxylic acids by direct polymerization in the presence of an antimony, tin or titanium catalyst. The patent does not, however, address the problem of aromatic hydroxy acid decarboxylation upon direct esterification, nor suggest any method for the direct formation of polyesters such as those disclosed in the aforenoted U.S. Pat. No. 4,161,470. Dihydric phenols and aromatic dicarboxylic acids are the only reactants used.
William W. Lowrance, Jr., in Tetrahedron Letters, No. 37, pp. 3453-3454 (1971), discloses that phenol esters such as phenyl benzoates can be prepared by direct esterification upon refluxing a solution containing phenol, benzoic acid and catalytic amounts of boric and sulfuric acids. The article does not pertain to the preparation of polyesters or to the direct reaction of aromatic hydroxy acids.
Higashi et al. in Journal of Polymer Science: Polymer Letters Edition, Vol. 18, 385-388 (1980), disclose that aromatic polyesters can be prepared directly from aromatic hydroxy acids such as p- and m-hydroxybenzoic acids, as well as a combination of dicarboxylic acids and bisphenols, by conducting the polycondensation in the presence of hexachlorocyclotriphosphatriazene in pyridine. The use of such a promoter, however, has many disadvantages, e.g., cost.
Thus, an easy and economically attractive process for preparing melt-processable, aromatic hydroxy acid comprising polyesters via the direct polycondensation thereof is heretofore unknown to the prior art and is a desideratum thereof.
Accordingly, it is an object of the present invention to provide a novel and more economically attractive process for preparing wholly aromatic polyesters of aromatic hydroxy acids.
It is another object of the present invention to provide a process for preparing aromatic polyesters which minimizes the expected decarboxylation of aromatic hydroxy acids and the adverse effects thereof.
Another object of the present invention is to provide a direct polycondensation process for preparing the polyesters claimed in U.S. Pat. No. 4,161,470.
Still another object of the present invention is to provide an efficient process for the direct polymerization of 6-hydroxy-2-naphthoic acid and p-hydroxybenzoic acid or other aromatic hydroxy acids.
These and other objects, as well as the scope, nature and utilization of the invention, will be apparent to those skilled in the art from the following description and the appended claims.